INFLUENCE OF NiSUBSTITUTION FOR Mn ON THE STRUCTURE AND IONIC CONDUCTIVITY OF LiNixMn2-xO4 SPINEL MATERIALS PREPAIRED BY THE SOL-GEL METHOD Ta Anh Tan 1 , Le Huy Son 1 , Dang Tran Chie
Trang 1INFLUENCE OF NiSUBSTITUTION FOR Mn ON THE STRUCTURE AND IONIC CONDUCTIVITY OF LiNixMn2-xO4 SPINEL MATERIALS
PREPAIRED BY THE SOL-GEL METHOD
Ta Anh Tan 1 , Le Huy Son 1 , Dang Tran Chien 2
1
Faculty of Natural Sciences,Hanoi Metropolitan University
2
Hanoi University of Natural Resources and Environment
Abstract: Electrode materials LiNixMn2-xO4 with (x = 0; 0.05; 0.1; 0.2) were synthesized
by the sol-gel method from lithium acetate, manganese acetate and nikel acetate The FE-SEMs show that the morphology of the material changes when the anealing temperature and the proportion replacement of Mn with Ni changes XRDs confirmed that the samples have LiNixMn2-xO4 spinel structures without any contaminants Lattice constants of the material increase with annealing temperature and decrease when Ni ratio substitution increases As the proportion replacement of Ni increases, the particle size of the LiNixMn2-xO4 samples decreases while the grain boundary changes from the rounded form at x = 0
to the form of sharp edges at x = 0.1 and 0.2 These results show the effect of nickel doping
on crystal stability The studies of impedance spectroscopy were applied to the bulk materials showing the Li+ ion conductivity of the material The results indicate that substituting Ni for Mn improved the conductivity of the materials tp = 19,773 × 10-5 S.cm-1 with x = 0.1, anealing temperature at 700 °C compared to tp = 0.111 × 10-5
S.cm-1 of the samples with x = 0
Keywords: Electrode materials, LiNixMn2-xO4, Liti-ion batteries, Ion conductivity, Ni
substitution
Email: tatan@daihocthudo.edu.vn
Received 05 December 2017
Accepted for publication 25 December 2017
1 INTRODUCTION
Cathode materials for lithium ion battery are based on three basic materials. Those are: i/ LiCoO2 layer structure (LCO); ii/ LiMn2O4 (LMO) spinel structure; iii/ LiFePO4 olivine structures (LFP). These are materials that exchange and store H+ and Li+ ions very well, and they are the basis materials used for making cathode electrodes in lithium ion batteries (LIBs).
Trang 2The most important thing is that ion-exchange electrode materials used in lithium ion batteries must simultaneously have high electron conductivity and conductivity. However, the recent works have shown that ionic conductivity of LiMn2O4 reached the value of
10-6 ÷ 10-10S.cm-1 [1]. This low value of ionic conductivity leads to weakness of the electrochemical activity and slowness of the flow rate of the battery cycling.
Thus, many studies have attempted to improve the ionic conductivity of the materials such as changing in the methods of fabrication; synthesis temperature or replacement of Mn
in LiMn2O4 with the metals (Li, Co, Ni, Al, Mg, Cr, Fe). These can improve the conductivity
of the materials. Among these materials, LiNixMn2-xO4 shows the best charging/discharging stability [2-6]. Although LiNixMn2-xO4 performs a good improvement in lithium ion conductivity and stability during charging/discharging but if a large amount of Ni substitution for Mn can significantly reduce power at 4 V. Therefore, most studies on LiNixMn2-xO4 have been limited with x ≤ 0.2 for stable crystalline structure and optimum electrochemical efficiency [7, 8].
There are many methods used for synthesis of the LiNixMn2-xO4 spinel such as: solid phase reaction method [9]; sol-gel method [10]; polime spray [11]; hydrothermal[12], etc… However, in this study, the LiNixMn2-xO4 spinel with (x = 0; 0.05; 0.1 and 0.2) were fabricated by the sol-gel method. This is a simple technology method, high economic efficiency and can be produced in large quantities.
2 EXPERIMENTAL
LiNixMn2-xO4 ion-conducting materials are made from lithium acetate, manganese acetate and nickel acetate based on the ratio of atomic composition Li: Ni: Mn = 1: x :2 - x with (x = 0; 0.05; 0.1 and 0.2). The initial materials were dissolved in deionized water at a certain ratio of solubility, then stirred at 80 °C for 10 hours (to get the gel formation, citric acid is added at a ratio of Li: Mn: citric = 1: 2: 3 and the pH is kept at 7 by addition of NH3) then a gel is formed. This gel is dried in air for 15 hours at the temperature of 120 °C. Finally, the materials were anealed at temperature of 300 °C; 500 °C; 700 °C; and 800 °C for 6 hours. The samples are denoted as in Tab 1.
The X-ray diffraction system D5005 SIEMEN with the CuKα emission source (λ = 1.5406Å) was used to investigate the structural characteristics of the materials. The constants
of the lattices and crystal structure parameters were determined by Sherrer mode and Unitcell software. Morphological characteristics were examined with scanning electron microscope FE-SEM HITACHI 4800.
Trang 3Tab 1 Symbol of LiNi x Mn 2-x O 4 with Ni substitution x = 0, 0.05, 0.1 and 0.2 at anealing
temperature of 300 °C, 500 °C, 700 °C and 800 °C
Sample symbol LiNi x Mn 2-x O 4 Temperature (C)
Impedance spectra were applied to investigate the
ionic conductivity of the samples via electrochemical
systems Autolab PSGTAT 100. The ionic conductivity of
the material is determined by the method of matching the
experimental results using the NOVA software. The
samples were prepared by pressing a 1cm- diameter pellet
with a pressure of 40 MPa/cm2. A Au electrode with a
diameter of 0.8 cm and a thickness of 1 μm was deposited
on both sizes of each sample using the vacuum evaporation
method. The pattern is described in fig.1.
Fig 1 Sample for impedance
spectrometry
Trang 43 RESULTS AND DISCUSSION
3.1 Effect of temperature on morphology of LiNi x Mn 2-x O 4 materials
Fig 2 SEM images of the LiNi x Mn 2-x O 4 materials with replacement ratio of Ni (x = 0) synthesized by the sol - gel methood and then annealed at 300 °C; 500 °C; 700 °C and
800 °C
Fig 3 SEM images of the LiNi x Mn 2-x O 4 materials with replacement ratio Ni ( x = 0.05;
0.1 and 0.2) annealed at 300 °C
Fig 4 SEM images of the iNi x Mn 2-x O 4 materials with replacement ratioof Ni ( x = 0.05;
0.1 and 0.2) annealed at 500 °C
Fig 5 SEM images of the LiNi x Mn 2-x O 4 materials with replacement ratio of Ni (x = 0.05;
0.1 and 0.2) annealed at 700 °C
Trang 5The images from Figures 2 to 6, are the SEM images of the LiNixMn2-xO4 material samples with different proportions of Ni molecules and at the anealing temperatureof 300 ° C; 500 °C; 700 °C and 800 °C. SEM images show that the morphology of the LiNixMn2-xO4 materials with the replacement ratio of Ni (x ranges 0 to 0.2) with annealing temperature at from 300 °C to 500 °C. The grain sizes change very little while being grouped into clusters. When the anealing temperature increases, the particles tend to separate, and at the temperature at from 700 °C to 800 °C the particle sizes increase very strongly. The average size of LiNixMn2-xO4 crystal particles calculated from the SEM image are shown in Tab 2. The above results show that the size of the crystalline particles depends strongly on the anealing process.
Tab 2 Everage value of LiNi x Mn 2-x O 4 particle annealed at different temperatures
LiNi x Mn 2-x O 4 T = 300 C T = 500 C T = 700 C T = 800 C
At temperatures below 500 °C, the grain size changes little when the annealing temperature between 300 °C and 500 °C. When the anneaing temperature is increased to 700
°C, particle size increases significantly and particles tend to separate. Especially at 800 °C the particles grow very fast and the size increasesseveral times. The rapid growth of particle size at temperatures of 700 °C and 800 °C is due to the formation of LiMn2O4 at about 700
°C (698 °C). This has been pointed out from the schema differential thermal analysis of the sample DTG and DTA of the LiMn2O3 [13] and it also explains why at 800 °C particle size increased several times compared to that at below 700 °C.
Fig 6 SEM images of the LiNi x Mn 2-x O 4 materials with replacement ratio Ni ( x = 0.05;
0.1 and 0.2) annealed at 800 °C
Trang 63.2 Effect of Ni replacement ratio on the morphology of the LiNi x Mn 2-x O 4
materials
SEM images (Figure 2 ÷ 6) show that at temperatures below 700 °C, replacement ratio
of Ni dose not impact to the crystal particle size (the sizes ranged from 40 nm to 60 nm). At
a temperature of 700 °C the particle size decreases, the grain boundary changes from round
to sharp when the replacement ratio of the Ni molecule increases. This demonstrates that the presence ofNi increases the stability of the spinel structure of the LiMn2O4 materials. It is perfectly suited to the study [2, 6]. In this work, Ni substituted for Mn reduces lattice distortion Jahn - Teller [14-16]. In other words, there was a substitution of Ni atoms for Mn atoms in the LiNixMn2-xO4 material produced by the sol-gel method.
3.3 Structural characteristics of LiNi x Mn 2-x O 4 materials
As known at room temperature, the LiMn2O4 spinel materials have a cubic structure with the space group Fd-3m, where the Li, Mn and O atoms respectively occupy positions 8a, 16d, and 32e [17]. Meanwhile lattice structure and the arrangement of atoms in the lattice can be shown as in Figure 7a and the formation of Li+ ion channel through the octahedral as presented in Figure 7b.
Fig 8 XRD spectra of LiNi x Mn 2-x O 4 materials with Ni substitution x = 0 (a) and 0.05 (b) synthesized by the sol-gel method at 300 °C; 500 °C; 700 °C and 800 °C.
Fig 7. Crystal structure of LiMn 2 O 4 materials (a) Illustrate the diffusion of Li + through location 16c (b) The black arrow indicates the diffusion path of the Li + ion.
Trang 7Fig 9 XRD spectra of LiNi x Mn 2-x O 4 materials with Ni substitution x = 0.1 (a) and 0.2 (b)
synthesized bythe sol-gel method at 300 °C; 500 °C; 700 °C and 800 °C.
Fig. 8 and 9 are the XRD spectra of G0, G1, G2 and G3 samples synthesized by the sol-gel method at 300 °C; 500 °C; 700 °C and 800 °C. At all Ni replacement ratioes and annealing temperatures, all diffraction peaks on the XRD spectra match a single JPCDS No 35-0782. This suggests that substituting Ni with proportions (x = 0 ÷ 0 .2) did not change the structure of LiNixMn2-xO4 materials compared to the structure of the original LiMn2O4 spinel materials. This result shows that Ni has replaced the Mn position in the crystal lattice.
In other words, LiNixMn2-xO4 materials have been successfully synthesized in which the substitution of Ni for Mn with the ratio of x = 0; 0.05; 0.1 and 0.2. However, X-ray diffraction is only sufficient to show that the vertices of the spinel material phase are formed without being able to show whether the material produced contains Ni ions. In order to obtain the proof of this substitution in the formulated samples, we proceeded to analyze the Raman spectra of Ni substitution samples for Mn.
Fig 10 Raman Spectra of G0-700 and
G2-700 (LiNi x Mn 2-x O 4 replacement ratio of Ni x = 0 (a) and x =0,1 (b).
Trang 8Fig.10 is the Raman scattering spectra of the samples G0-700 and G2-700. Fig.10a, the Raman spectra of spinel LiMn2O4 shows a broad and strong region (Ranging Mn-O stretching) at ~ 620 cm-1 accompanied by a small peak at ~ 580 cm-1.
They are closely related to the octahedral MnO6 and the oxidation state of Mn, respectively called A1g and F( ) [18]. The expansion of the A1g region is due to the small difference in the octahedral Mn4+O6 octahedral structure and the octahedral Mn3+O6 is partially distorted in LiMn2O4. Its intensity depends on the concentration of Mn4+ in the material and reflects the average oxidation state of Mn. For this reason, according to Yingjin Wei and colleagues [18] the regions A1g and F( ) are not separated in unmodified LiMn2O4 because the concentrations of Mn3+ and Mn4+ are equal in the material. Then F( ) is clearly distinguished by the substitution of Ni and the sequence A1g becomes distinct and sharper from the region F( ). The change in A1g and F( ) with the replacement of Ni is consistent with the increase in Mn4+ concentration as well as the increase of Mn oxidation state in LiNixMn2-xO4. Peak F( ) derives primarily from the oscillation of the Mn4+- O bond. Its intensity depends on the concentration of Mn4+ in the medium reflecting the oxidation state
of Mn in Figure 10b.
3.4 Effects of temperature and Ni substitution ratio on the lattice constants of the materials
Fig 11 Dependence of the lattice constants on the Ni (a)
ratio and the annealing temperature (b).
Fig.11 showed the dependence of the lattice constant on the annealing temperature. (Fig.11b) and the substitution ratio of Ni for Mn (Fig.11a). As can be seen from Fig.11b,
Trang 9lattice constant of the materials increased slightly,about 0.015 Å when the annealing temperature increased from 300 °C to 800 °C. The increase of crystalline lattice constant of the LiNixMn2-xO4 materials is explained by the transition of Mn4+ to Mn3+ (LS or HS) and the transition of Mn3+ (LS) to Mn3+ (HS) as the annealing temperature increases. Fig. 11 a shows that the crystal lattice constant of the material decreases to 0.023 Å when the replacement ratio of Ni increases from x = 0 to x= 0.2. From XRD spectra shown in fig. 8 and 9, one can find in all the samples, the substitution ratio of Ni for Mn increases, the diffraction peak at angle 20 is higher. This suggests that the lattice constant of the material
is reduced when the replacement ratio of Ni increases [18]. It is explained that when the replacement ratio of Ni in the LiNixMn2-xO4 materials increases leading to increase of the
Mn4+ concentration. The ionic radius of Mn4 + (r = 0.53 Å) is much smaller than Mn3+ (r = 0.645 Å). The ionic radius of Ni3+ (r = 0.56 Å) is smaller than the radius of Mn3+ ion (r = 0.645 Å) [19]. When Mn3+ion is replaced by Ni3+ ion, Mn - O distance was reduced. Oxygen defect spaces at high annealing temperatures were also reduced [18].
3.5 Li + ion conductivity of the materials system
Fig.12 showed the typical Nyquist plot in the complex plane presented the imaginary part Z" depends on the real part Z'of LiNixMn2-xO4 materials at room temperature. The impedance spectra consist of two semicircular regions. A semicircular in the high frequency region from 1MHz to a few tens Hz, they are attributed to the lithium ion conduction in the particle and a semicircular in the low frequency region is attributed to the ionic conduction
at the grain boundary [19, 20]. Total resistance (Rb + Rgb) and bulk resistor (Rb) of the samples correspond to block point on the right and the left of the semicircular with the real axis in the schema. The value of the grain boundary resistance (Rgb) is reflected by the difference between (Rb + Rgb) and Rb.
Fig 12 Nyquist diagram of LiNi x Mn 2-x O 4 doped Ni (x = 0; 0.1 and 0.2) synthesized by Sol-gel at
700 °C (a) and block points of the two regions of the semicirculars (b)
Trang 10Fig.12 showed that when synthesis temperature or ratio of Ni change, it leads to changing block point in both of the low frequency and high frequency on the Nyquits diagram. This shows that the Rb and Rgb resistors are all change and the particle conductivity
b and grain boundary conductivity gb of the materials will depend on both synthesis temperature and replacement ratio of Ni with Mn.
The dependence of the conductivity on the nickel substitution ratio and the synthetic temperature are shown in Fig.13 and 14.
The calculation results of the lithium ion conductivity of LiNixMn2-xO4 materials showed that the lithium ion conductivity changed when the nickel replacement ratio and annealing temperature changed. In particular, the LiNixMn2-xO4 materials have a nickel
replacement ratio of x = 1 annealed at 700 °C gives the best lithium ion conductivity (G2-700 sample) with the largest total conductivity tp = 19,773×10-5 S.cm-1. On the contrary, samples G0-500 with the smallest conductivity tp = 0,116×10-5 S.cm-1.
Fig.13 Influence of synthesis temperature on ion conductivity of LiNi x Mn 2-x O 4 synthesized by
sol-gel method (G0, G1, G2 and G3).
Fig.14. Influence of the mixing ratio of Ni on the ion conductivity of the LiNi x Mn 2-x O 4 materials
synthesized by the sol-gel method (G0, G1, G2 and G3 ).
4 CONCLUSION
LiNixMn2-xO4 materials have been successfully fabricated by the sol-gel method. Particle size increases as the annealing temperature increases and decreases when the